Hip stem prosthesis

ABSTRACT

A hip stem prosthesis is provided for treating a deficient hip joint.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.14/510,695, filed on Oct. 9, 2014, now issued as U.S. Pat. No.9,636,227, which is a continuation of U.S. patent application Ser. No.13/465,172, filed on May 7, 2012, now issued as U.S. Pat. No. 8,858,646,which is a continuation of U.S. patent application Ser. No. 12/880,365,filed on Sep. 13, 2010, now issued as U.S. Pat. No. 8,206,455, which isa continuation of U.S. patent application Ser. No. 11/063,030, filed onFeb. 22, 2005, now issued as U.S. Pat. No. 7,842,096, the benefit ofpriority of each of which is claimed hereby, and each of which areincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The invention relates to orthopedic implants. In particular, thisinvention relates to hip stem prostheses.

BACKGROUND

Total hip arthroplasty is often used to restore function to a diseasedor injured hip joint. Positions and directions relative to the hip jointmay be described in terms of proximal being nearer the hip joint, distalbeing further from the hip joint, anterior being nearer the front of thebody, posterior being nearer the back of the body, medial being nearerthe centerline of the body, and lateral being further from the centerline of the body. In total hip arthroplasty, the surfaces of the femurand pelvis are cut away and replaced with substitute implants. In atypical case, the implants include a hip stem component, a femoral headcomponent, and an acetabular component.

The femoral bone is prepared by creating an opening down theintramedullary canal into the femoral bone along an axis from a proximalposition at the upper end of the femur toward a distal position at thelower end of the femur. The pelvis is prepared by reaming theacetabulum. The implants may be placed directly in contact with theprepared bone surfaces for bony fixation of the implant. Alternatively,bone cement may be introduced into the prepared canal and acetabulum sothat it hardens around and locks the components in place.

The hip stem component includes a stem portion extending down into theintramedullary canal of the femur and a neck portion extending away fromthe femur to support the femoral head component.

A recent development is the use of minimally invasive surgicaltechniques in which the bone is prepared and the implants insertedthrough small incisions that cause less trauma to surrounding musclesand other soft tissues such that the patients recovery is faster. Suchminimally invasive surgical techniques can be challenging due to thedifficulty in visualizing the surgical cavity and maneuvering theinstruments and implants within the tight confines of the incision.

SUMMARY

The present invention provides a hip prosthesis for insertion into afemur having a proximal end adjacent a hip joint, a distal end adjacenta knee joint, an anterior side, a posterior side, a medial side, alateral side, and an intramedullary canal.

In one aspect of the invention, the prosthesis includes a stem having aproximal end, a distal end, and a longitudinal axis. The stem includesanterior and posterior locking surfaces which each diverge from the stemaxis proximally at an angle greater than 3 degrees. The stem furtherincludes a shank portion extending distally from adjacent the anteriorand posterior locking surfaces and converging at an angle distallytoward the stem axis

In another aspect of the invention, the prosthesis comprises a lateralsurface which tapers both proximally and distally toward the stem axisto create a relief zone proximally and transitioning to a conicaltapering surface distally.

In another aspect of the invention, the lateral surface diverges fromthe stem axis distally to an inflection point which is the most lateralpoint on the lateral surface and the lateral surface converges towardthe stem axis distally from the inflection point.

In another aspect of the invention, the prosthesis includes anterior andposterior planar faces between the anterior and posterior lockingsurfaces and the shank portion. The anterior and posterior planarsurfaces being angled relative to the stem axis at an angle shallowerthan the angle of the shank portion.

BRIEF DESCRIPTION OF THE DRAWINGS

Various examples of the present invention will be discussed withreference to the appended drawings. These drawings depict onlyillustrative examples of the invention and are not to be consideredlimiting of its scope.

FIG. 1 is a perspective view of an illustrative hip stem according tothe present invention;

FIG. 2 is a front elevation view of the hip stem of FIG. 1;

FIG. 3 is a section view taken along line 3-3 of FIG. 2;

FIG. 4 is a is a section view taken along line 4-4 of FIG. 2;

FIG. 5 is a is a section view taken along line 5-5 of FIG. 2;

FIG. 6 is a is a section view taken along line 6-6 of FIG. 2;

FIG. 7 is a is a section view taken along line 7-7 of FIG. 2;

FIG. 8 is a is a section view taken along line 8-8 of FIG. 2;

FIG. 9 is a is a section view taken along line 9-9 of FIG. 2;

FIG. 10 is a is a section view taken along line 10-10 of FIG. 2;

FIG. 11 is a side elevation view of the hip stem of FIG. 1; and

FIG. 12 is a section view taken along line 12-12 of FIG. 11.

DESCRIPTION OF THE ILLUSTRATIVE EXAMPLES

FIGS. 1-12 illustrate an illustrative hip stem 10 having a proximal end12, a distal end 14, and a longitudinal axis 16 therebetween. The stem10 includes anterior, posterior, medial, and lateral sides 18, 20, 22,24. A neck 1 extends at an angle from the proximal end 12 of the stem 10to support a femoral head component (not shown) in articulating with thepelvis. The present investigators have found that hip stem fixation isenhanced by designing the stem differently in each of three generalportions including a proximal locking portion 40, a transition portion80, and a distal shank 100.

The proximal locking portion 40 advantageously includes anterior andposterior locking surfaces 42, 44 that diverge from the stem axis 16proximally as best seen in FIG. 10. The locking surfaces 42, 44 arepreferably flat planar surfaces that diverge equally from the stem axis16 to provide initial fixation. Each surface 42, 44 forms an angle withthe stem axis of from 3 to 10 degrees; preferably from 5 to 10 degrees;and more preferably 7 degrees. The included angle 46 is therefore from6-20 degrees; preferably from 10-20 degrees; and more preferably 14degrees. At angles greater than 3 degrees, and especially within thepreferred ranges, the locking surfaces 42, 44 form an anteroposteriorwedge that provides rigid initial fixation and which is highlyresistance to subsidence distally into the femur. The locking surfaces42, 44 also result in axial loads on the stem 10 being advantageouslytransferred to the femur with a relatively larger axial component and arelatively smaller radial component than would be the case with smallerangles such that the femur axially loaded proximally with reducedpotential for femoral fracture. Finally, the locking surfaces 42, 44result in a definitive stopping point as a surgeon is driving the stem10 into a femur which can be felt indicating that the stem 10 is fullyseated. The locking surfaces 42, 44 converge medially 47 as best seen inFIG. 9 to fit the proximal femoral geometry.

The proximal locking portion also advantageously includes a hyperbolicshaped flat relief surface 48 to better avoid stem 10 impingement withthe bone during stem insertion. In particular, the relief surface 48helps to avoid impingement with the greater trochanter. The surface 48is created by angling the proximal portion of the lateral side 24 intoward the stem axis 16. The relief surface 48 transitions from a flatsurface to a tapering cone extending distally downward toward the tip ofthe stem 10. Referring to FIG. 2, the relief surface 48 forms a surfacethat diverges from the stem axis 16 distally to an inflection point 50which is the most lateral point on the lateral surface 24 andcorresponds to the apex of the hyperbolic shaped surface 48. Thus, bymoving the lateral-most point on the lateral surface 24 distally awayfrom the proximal end 12, the present investigators have improved theinsertion characteristics of the stem 10, From the inflection point 50to the distal shank portion, the lateral surface 24 converges toward thestem axis 16 at a 2.5 degree angle.

The proximal locking portion 40 extends from 10 to 30 percent of theoverall stem length measured between the proximal and distal ends 12, 14of the stem 10. Preferably the proximal locking portion 40 extends from15 to 25 percent. More preferably, the locking portion 40 extendsdownwardly from the proximal end 12 by 20 percent of the stem length.Preferably, the proximal locking portion 40 includes a porous surfacegeometry 60 to promote bony ingrowth for long termfixation. In theillustrative hip stem 10, the porous surface 60 covers the proximallocking portion 40 in a continuous layer over the anterior, posterior,medial, and lateral sides 18, 20, 22, 24. The porous surface 60 extendsdistally into the transition portion 80 on the medial side 22 and wrapspartway over the anterior and posterior sides 18, 20. Preferably, theporous surface includes a tantalum metal porous surface having astructure similar to that of natural trabecular bone. Such a material isdescribed in U.S. Pat. No. 5,282,861 entitled “OPEN CELL TANTALUMSTRUCTURES FOR CANCELLOUS BONE IMPLANTS AND CELL AND TISSUE RECEPTORS”,issued to R. B. Kaplan and assigned to Ultramet. The material isfabricated of tantalum using vapor deposition. This material has beensold by Implex Corporation of Allendale, N.J., under the tradenameHEDROCEL. Zimmer, Inc., with manufacturing facilities in Warsaw, Ind.,sells a line of surgical implants incorporating this trabecular metaltechnology. The trabecular metal consists of interconnecting poresresulting in a structural biomaterial that is 80% porous and whichallows much greater bone ingrowth compared to conventional porouscoatings and much greater shear strength. In addition, the trabecularmetal possesses a high strength-to-weight ratio. The material isproduced by vapor depositing tantalum on an open celled porous matrix.

The neck 17 advantageously includes opposed flat surfaces 19 formed onit between the proximal end 12 of the stem 10 and the free end 21 of theneck 17. The flat surfaces 19 provide clearance for increasedarticulation of the stem relative to an acetabular component (notshown). The flat surfaces 19 further provide an engagement surface foran instrument to grip the neck 17.

The transition portion 80 of the stem 10 advantageously includesanterior and posterior faces 82, 84 to provide clearance between thestem and the cortical bone of the intramedullary canal of the femurduring minimally invasive surgical procedures. In a minimally invasivehip procedure, soft tissues tend to push the stem 10 into the sides ofthe intramedullary canal such that the stem insertion forces are notacting straight down into the canal. These off axis forces can lead tofemoral fractures. The anterior and posterior faces 82, 84 of thepresent invention relieve initial insertion hoop stresses making steminsertion easier and safer.

The anterior face 82 lies between the anterior locking surface 42 andthe shank portion 100 and the posterior face 84 between the posteriorlocking surface 44 and the shank portion 100. The faces 82, 84 furtherlie between the medial and lateral sides 22, 24. The anterior andposterior faces 82, 84 are preferably angled relative to the stem axisat an angle shallower than the angle of the anterior and posteriorlocking surfaces 42, 44 and preferably at an angle shallower than thetaper of the shank portion 100 which will be discussed below. Preferablythe faces 82, 84 are each angled relative to the axis 16 at an angleless than 3 degrees; more preferably the faces 82, 84 are parallel tothe axis 16.

The distal shank portion 100 generally forms a tapering cone having anincluded angle of 3 degrees. However, while maintaining the 3 degreetaper, the shank 100 advantageously transitions from a generallycylindrical cross section as shown in FIG. 6 to a polygonal crosssection as shown in FIGS. 7 and 8. The polygonal sections includepolygonal faces 102 connected by vertices 104. The faces 102 provideclearance for the shank 100 so that only the vertices 104 contactcortical bone distally to provide stem centralization while easinginsertion. The vertices 104 form a press fit with the distalintramedullary canal and may be size to bite into the bone slightly toprovide a degree of rotational resistance to the stem 10. The faces 102also provide a space between the shank 100 and the intramedullary canalto relieve pressure from body fluids within the canal to minimize thepossibility of an embolism upon insertion of the stem 10 into the canal.

Although examples of a hip stem prosthesis and its use have beendescribed and illustrated in detail, it is to be understood that thesame is intended by way of illustration and example only and is not tobe taken by way of limitation. Accordingly, variations in andmodifications to the hip stem prosthesis and its use will be apparent tothose of ordinary skill in the art, and the following claims areintended to cover all such modifications and equivalents.

What is claimed is:
 1. A hip prosthesis having a medial side, a lateralside, an anterior side, and a posterior side, the hip prosthesiscomprising: a neck; and a stem defining a longitudinal axis and having aproximal portion, a distal portion, and a transition portion extendingbetween the proximal portion and the distal portion of the stem, theproximal portion of the stem including an anterior surface, a posteriorsurface, and a lateral surface, the lateral surface of the proximalportion having a lateral most point at which the lateral surfacetransitions from a flat relief surface to a conical tapering surface,the flat relief surface angling toward the longitudinal axis of the stemin a proximal direction from the lateral most point and the conicaltapering surface angling toward the longitudinal axis of the stem in adistal direction from the lateral most point, wherein the distal portionof the stem has a proximal cross-section defining a circle and a distalcross-section defining a polygon.
 2. The hip prosthesis of claim 1,wherein the anterior surface and the posterior surface of the proximalportion diverge away from the longitudinal axis of the stem in adistal-to-proximal direction along the stem so as to form a firstincluded angle of between six degrees and twenty degrees, and whereinthe distal portion of the stem includes a shank portion with a taperthat adjoins the transition portion, the transition portion of the stemhaving an anterior face and a posterior face that form a second includedangle that is less than the first included angle.
 3. The hip prosthesisof claim 2, the second included angle is less than the taper of theshank portion that adjoins the transition portion.
 4. A hip prosthesishaving a medial side, a lateral side, an anterior side, and a posteriorside, the hip prosthesis comprising: a neck; and a stem defining alongitudinal axis and having a proximal portion, a distal portion, and atransition portion extending between the proximal portion and the distalportion of the stem, the proximal portion of the stem including ananterior locking surface, a posterior locking surface, and a lateralsurface, the anterior locking surface and the posterior locking surfaceof the proximal portion diverging away from the longitudinal axis of thestem in a distal-to-proximal direction along the stem so as to form afirst included angle of between six degrees and twenty degrees, thelateral surface of the proximal portion having a lateral most point, thelateral surface including a flat relief surface that angles toward thelongitudinal axis of the stem in a proximal direction from the lateralmost point, the lateral surface also angling toward the longitudinalaxis of the stem in a distal direction from the lateral most point,wherein the distal portion of the stem includes a shank portion with ataper that adjoins the transition portion, the transition portion of thestem having an anterior face and a posterior face that form a secondincluded angle that is less than the first included angle, and whereinthe second included angle is less than the taper of the shank portionthat adjoins the transition portion.
 5. The hip prosthesis of claim 4,wherein the distal portion of the stem has a proximal cross-sectiondefining a circle and a distal cross-section defining a polygon.
 6. Thehip prosthesis of claim 4, wherein the anterior locking surface and theposterior locking surface of the proximal portion converge medially. 7.The hip prosthesis of claim 6, wherein the stem has a continuous porousouter layer that completely surrounds the stem along the mediallyconverging anterior and posterior locking surfaces such that a crosssection of the stem through the medially converging anterior andposterior locking surfaces intersects the porous outer layer on themedial, lateral, anterior, and posterior sides of the hip prosthesis. 8.The hip prosthesis of claim 4, wherein the stem has a stem length andthe proximal portion of the stem extends from 10 percent to 30 percentof the stem length.
 9. The hip prosthesis of claim 4, wherein the distalportion of the stem includes a proximal section and a distal section,the proximal section adjoining the transition portion of the stem andhaving a cross-section that defines a circle, the proximal sectiontransitioning distally into the distal section, the distal sectionhaving a non-circular cross-section.
 10. The hip prosthesis of claim 4,wherein the transition portion of the stem has a non-circularcross-section.
 11. The hip prosthesis of claim 4, wherein the anteriorlocking surface and the posterior locking surface of the proximalportion define a maximum width of the hip prosthesis measured betweenthe anterior and posterior sides of the hip prosthesis.
 12. A hipprosthesis having a medial side, a lateral side, an anterior side, and aposterior side, the hip prosthesis comprising: a neck; and a stemdefining a longitudinal axis and having a proximal portion, a distalportion, and a transition portion extending between the proximal portionand the distal portion of the stem, the proximal portion of the stemincluding an anterior surface, a posterior surface, and a lateralsurface, the lateral surface of the proximal portion having a lateralmost point at which the lateral surface transitions from a flat reliefsurface to a conical tapering surface, the flat relief surface anglingtoward the longitudinal axis of the stem in a proximal direction fromthe lateral most point and the conical tapering surface angling towardthe longitudinal axis of the stem in a distal direction from the lateralmost point, wherein the anterior surface and the posterior surface ofthe proximal portion diverge away from the longitudinal axis of the stemin a distal-to-proximal direction along the stem so as to form a firstincluded angle of between six degrees and twenty degrees, and whereinthe distal portion of the stem includes a shank portion with a taperthat adjoins the transition portion, the transition portion of the stemhaving an anterior face and a posterior face that form a second includedangle that is less than the first included angle, and wherein the secondincluded angle is less than the taper of the shank portion that adjoinsthe transition portion.
 13. The hip prosthesis of claim 12, wherein thestem has a stem length and the proximal portion of the stem extends from10 percent to 30 percent of the stem length.
 14. A hip prosthesis havinga medial side, a lateral side, an anterior side, and a posterior side,the hip prosthesis comprising: a neck; and a stem defining alongitudinal axis and having a proximal portion, a distal portion, and atransition portion extending between the proximal portion and the distalportion of the stem, the proximal portion of the stem including ananterior locking surface, a posterior locking surface, and a lateralsurface, the anterior locking surface and the posterior locking surfaceof the proximal portion diverging away from the longitudinal axis of thestem in a distal-to-proximal direction along the stem so as to form afirst included angle of between six degrees and twenty degrees, thedistal portion of the stem including a proximal section and a distalsection, the proximal section adjoining the transition portion of thestem and having a cross-section that defines a circle, the proximalsection transitioning distally into the distal section, the distalsection having a non-circular cross-section, wherein the distal sectionof the distal portion of the stem has a cross-section that defines apolygon, wherein the transition portion of the stem has a non-circularcross-section, and wherein the proximal section of the distal portion ofthe stem has a first degree of taper and the distal section of thedistal portion of the stem has a second degree of taper that is the sameas the first degree of taper.
 15. The hip prosthesis of claim 14,wherein the lateral surface of the proximal portion angles toward thelongitudinal axis of the stem in the distal-to-proximal direction. 16.The hip prosthesis of claim 14, wherein the lateral surface of theproximal portion has a lateral most point, the lateral surface anglingtoward the longitudinal axis of the stem in both a proximal directionfrom the lateral most point and a distal direction from the lateral mostpoint.
 17. The hip prosthesis of claim 14, wherein the lateral surfaceis oriented more vertically than horizontally such that, when the hipprosthesis is implanted, the lateral surface faces more in a lateraldirection than in a proximal direction.
 18. The hip prosthesis of claim14, wherein the stem has a stem length and the proximal portion of thestem extends from 10 percent to 30 percent of the stem length.